Manufacturing Stainless Steel

Transcription

1 Manuf ac t ur i ng St ai nl sst eel How dot hedi f f er entpr oc s af f ec tt hec or r os i onr i s t anc e?

2 Manufacturing Stainls Steel How do the different procs affect the corrosion ristance? No matter how the stainls steel is used, a certain degree of manufacturing is required. This may include cutting, bending, welding or grinding; however, no matter what we do, the corrosion ristance of the steel is affected. When the stainls steel leav the factory, it is perfect. From this moment, its corrosion ristance is at its bt, and the vast majority of manufacturing procs affect the ristance in a negative way. Most procs will tend to weaken the corrosion ristance, and, consequently, all manufacturing should be performed in such a way that the negative effect is as small as possible. If this is not possible, the manufacturing should be followed by a proper mechanical and/or chemical surface treatment. Fig. 1 Compared to other methods of manufacturing, welding impli the largt risk of introducing flaws and weakns. The risk of problems is huge, if things are not done according to the book. Welding (SS&C, Chapter 10.1) One of the most severe procs is welding. Apart from introducing a second phase (the filler metal), the steel is subject to a very powerful heat treatment, which may affect the corrosion ristance negatively in at least four negative ways: a) Geometric flaws, b) Sensitisation and formation of intermetallic phas, c) Heat tintings, and d) Tensile strs. Manufacturing stainls steel 1

3 The first (a) is related to the physical appearance of the weld, while the other three (b-d) are caused by the immense heat transfer during the welding procs. The risks connected to the geometry of the weld is often taken care of by using an over-alloyed filler metal. Most frequently, the defect is a matter of crevic, such as por, lack of bonding, incorrect burn-through and others, and the main corrosion risk will be crevice corrosion. A rule of the thumb stat that CC occurs at a temperature C below the critical pitting temperature (CPT). To cope with this, crevic should be completely avoided below the water line (= intensified control), or a better steel with a higher PREN should be chosen. Thereby, a larger safety margin is induced allowing a few more defects (SS&C, Table 6.1). Beware that the risk of bacterial growth cannot be coped with by increasing the corrosion ristance of the filler metal. Here, a perfect geometry is necsary to avoid the possible growth sit. Heating the steel to a temperature in between 500 and 850 ºC, an inevitable phenomenon close to the welding zone, impli a risk of formation of harmful chromium carbid, sensitisation (b). This do not happen in the weld itself, but rather in the Heat Affected Zone (HAZ), close by. Normally, this problem is greatt when welding thick steel plat, and in practice, one can cope with it by choosing low-carbon steel (4306, 4307 or 4404), or titanium stabilized steel (4541 or 4571). A related phenomenon is the formation of harmful intermetallic phas such as the sigma (Cr-Fe) or the ksi phase (Cr-Mo). This problem is particularly big when welding high-alloyed super duplex steels (i.e. 4410, duplex 2507), high-alloyed austenit (i.e. 4547) and the high-end ferritic steel typ (i.e. 4509, 4526 and 4521). C D C A F B F E D G G Sketch of a weld seam with all the possible problems highlighted:. A: The base steel; B: Weld seam; C: Natural oxide film; D: Heat tinting; E: Dechromed layer (right underneath the heat tinting); F: Heat Affected Zone (HAZ); G: Por, lack of binding etc. (crevic). Illustration from SS&C. Manufacturing stainls steel 2

4 Heat Tintings (SS&C, Chapter ) At least just as harmful is the bluish or yellowish heat tinting (c), which is formed on the steel surface during welding. The discolorations are caused by a warm oxidation of the steel surface and consist of thick oxid of mainly chromium and iron. If left untreated, the layers imply a significant loss of corrosion ristance, and the more oxygen in the gas, the darker the heat tintings (Fig. 4) and the more severe is the loss of corrosion ristance. The problem with the heat tintings is caused by the fact that the concentration of the useful chromium is increased as compared to the base material. In turn, this impli the formation of a thin de-chromed layer right below, subsequently implying an increased risk of of mainly pitting corrosion and penetration. Fig. 3 Gennemgående grubetæring set indefra i et 6 -rør med påsvejste 1 studs. Korrosionen er opstået pga. for meget ilt i baggassen, hvilket har svækket stålets PREN. Foto fra RS-FP. To get the bt rults, all welding should be carried out in a completely oxygen-free atmosphere. This requir a super gas cover (see FORCE Technology Reference Atlas, Publ SS&C, Ch ). Unfortunately, this is quite expensive, and a more feasible and economical way to cope with the risk of weld is to accept a slightly higher degree of heat tinting (i.e. a certain level of bluish discolouring) and later to remove the layer by pickling or a combination in between grinding and a subsequent pickling or passivation. Removing the heat tinting by a glass blasting is ls dirable as the heat tinting and the de-chromed layers will be mashed into the surface rather than being removed. Prior to the glass blasting, a proper pickling will do the job. Finally, any welding procs impli the formation of tensile strs (d), which will increase the risk of strs corrosion cracking (SCC, SS&C Ch. 6.4). As removal of the strs through a proper heat-treatment is normally not feasible, this problem should be taken care of in the dign phase by choosing a steel type posssing a sufficient ristance towards SCC. Fighting SCC by hoping to reduce the level of tensile strs is not recommended. Manufacturing stainls steel 3

5 Purge gas Argon Formier ppm oxygen (O 2) in the purge gas Fig. 4 Discolourations/heat tintings on the inside of dairy tub (63 1,6 mm) after TIG welding using varying content of oxygen in the pure gas. It is clear that an increasing oxygen content impli an increasing degree of heat tintings (left to right). Two different gas typ were used, and note that the active Formier gas (N 2/H 2, 90-10; lower seri) is better in order to prevent the heat tintings (same oxygen but ls discolouration) than the passive argon (Ar, top row). Fig. 5 Close-up photo showing a weld spatter on a stainls steel surface. Instead of a corrosion ristant free surface, the steel has become a heat-tinted crevice. The reduction in corrosion ristance is substantial. Spatter from angular cutters have the same sad effect. Manufacturing stainls steel 4

6 Cutting, Sawing and Others (SS&C, Chapter 10.2) Due to the risk of heat tinting, the most dangerous methods are the hot on. A hot classic is the angular cutter, which, apart from producing a rough and uneven surface, giv rise to a spray of hot particl. The have a nasty tendency to stick to stainls steel surfac, and the rult is heat tinted crevic, a very sad combination implying a severe loss of corrosion ristance. Though not foolproof, a way to cope with the problem is to remove the spray particl carefully with i.e. a chisel or a screw driver, if necsary grind carefully and finish off with a pickling. Even the cold cutting procs may affect the corrosion ristance in a negative way. The centre of the steel normally contains a larger concentration of harmful inclusions and segregations than the surface, and thus the centre of even thin sheets is ls corrosion ristant than the surface. This inevitable effect originat from the making of the steel at the steel works. When the steel solidifi, it tak place from the outside and inwards pushing the insoluble impuriti towards the centre of the slab. Hot and cold rolling the slab from a thickns of 300 mms to, say, ls than 1 mm still maintains the impuriti in the centre, and cutting the steel expos the impuriti, thereby compromising the corrosion ristance of the steel. As above, a subsequent chemical treatment (such as a pickling) will minimize the problem (SS&C, Chapter 12.1). Iron Contamination, Handling and Transportation (SS&C, Chapter 10.3) A particular risk when dealing with stainls steel is iron contamination originating from using the same tools and equipment for handling mild steel along with stainls steel. Using the same trucks, the same fork lifts or the same machinery may transfer minor amounts of mild steel or rust onto the stainls steel. Apart from looking ugly, the contaminations may cause corrosion of the stainls steel itself. Fig. 6 A fine example of severe mistreatment of stainls steel: A couple of severe scratch, prumably made by contact with mild (carbon) steel. Apart from the physical scratch themselv, the iron particl happily rust, thereby creating reddish-brown ferric oxid. Neither make the customer happy. Manufacturing stainls steel 5

7 Iron contaminations may be removed chemically; however, preventing the whole thing from happening is at least as effective. In particular, it s important only to use tools which have not been used for mild steel. This includ anything from the rolling and cutting tools to the forks of the fork lift. Please note that the tools themselv are frequently made of non-stainls high-strength carbon steel or low-grade martensitic stainls steels. Such steel grad are very hard and do not imply any risk of iron contaminations from the tools. The risk com from the soft steels which have been manufactured with the hard tools. Further advice is given in SS&C, Chapter A nasty example of carbon steel particle, having been mashed into the stainls steel surface during cold forming. The iron particle must have been very hard, as it has been prsed into the stainls steel, and even though a pickling will remove the iron contamination, the hol remain. Even if the tools are separated, another risk is the transfer of metal dust from the grinding of mild steel onto the stainls steel further down the alley. This can only be prevented by keeping the production of mild steel and stainls steel separated completely, preferably in two separate buildings. If this cannot be arranged, a chemical post-treatment is mandatory. All text and photos are and may only be used after a previous written consent from Damstahl. All referenc are Stainls Steel & Corrosion (SS&C; Claus Qvist Jsen, Damstahl; Oct. 2011) or Stainls Steel for Hygienic Equipment in Food/Pharma (SS-FP; Claus Qvist Jsen and Erik-Ole Jensen, Damstahl; Oct. 2014). Apart from Danish, both books are available in English and Swedish, and SS&C even in German. All books are ordered through. Manufacturing stainls steel 6